Ultrafast laser-scanning time-stretch imaging at visible wavelengths

Abstract

Optical time-stretch imaging enables the continuous capture of non-repetitive events in real time at a line-scan rate of tens of MHz—a distinct advantage for the ultrafast dynamics monitoring and high-throughput screening that are widely needed in biological microscopy. However, its potential is limited by the technical challenge of achieving significant pulse stretching (that is, high temporal dispersion) and low optical loss, which are the critical factors influencing imaging quality, in the visible spectrum demanded in many of these applications. We present a new pulse-stretching technique, termed free-space angular-chirp-enhanced delay (FACED), with three distinguishing features absent in the prevailing dispersive-fiber-based implementations: (1) it generates substantial, reconfigurable temporal dispersion in free space (>1 ns nm−1) with low intrinsic loss (<6 dB) at visible wavelengths; (2) its wavelength-invariant pulse-stretching operation introduces a new paradigm in time-stretch imaging, which can now be implemented both with and without spectral encoding; and (3) pulse stretching in FACED inherently provides an ultrafast all-optical laser-beam scanning mechanism at a line-scan rate of tens of MHz. Using FACED, we demonstrate not only ultrafast laser-scanning time-stretch imaging with superior bright-field image quality compared with previous work but also, for the first time, MHz fluorescence and colorized time-stretch microscopy. Our results show that this technique could enable a wider scope of applications in high-speed and high-throughput biological microscopy that were once out of reach.

Time-stretch microscopy on a DVD for high-throughput imaging cell-based assay

“…We also note that this spinning-disc assay platform can be compatible with not only bright-field and quantitative phase imaging, but also fluorescence imaging. For instance, a recent demonstration of ultrafast all-optical laser-scanning fluorescence imaging at a line-scan rate of > 1 MHz, based on a concept of free-space angular-chirp-enhanced delay (FACED) [27], could be a viable option to be combined with the current high-speed spinning assay platform to enrich fluorescence-image-based single-cell analysis, such as CTC detection with multi-color fluorescence staining [28].…”

A high‐throughput all‐optical laser‐scanning imaging flow cytometer with biomolecular specificity and subcellular resolution

“…A relay lens system and an objective lens (×40, numerical aperture (NA) = 0.66) are configured as an inverted microscope and are used to project the virtual sources onto the sample plane—resembling the ultrafast laser line‐scanning action, that is, pulses from different virtual sources arrive at the sample plane at different time points. The temporal delay between adjacent virtual sources is given by τ ≈ 2 S / c , where S is the mirror separation and c is the speed of light in free space . Without mechanical beam steering, the line‐scan rate is governed by the laser repetition rate (20 MHz).…”

“…The pixel size along y axis ( Δy ), that is, the microfluidic flow axis, are defined based on the microfluidic flow speed ( v ) and the repetition interval of the pulsed laser ( Δt ), that is, Δy = vΔt . As the temporal delay between adjacent virtual sources results in the modulation of the captured waveform , modulation peaks are detected and their amplitudes are assigned as the image pixel brightness. The extracted 2D image is normalized by the line scan background to form a bright‐field (BF) image in which the image contrast originates from light absorption and scattering of the cell.…”

“…To this end, we recently developed an ultrafast laser scanning imaging technique termed free‐space angular‐chirp‐enhanced delay (FACED) imaging . FACED creates a spatiotemporally encoded virtual source array in free space, which behaves as an all‐optical laser scanner without mechanical or active beam steering.…”

Ultrafast cell edge detection by line‐scan time‐stretch microscopy

“…A new pulse‐stretching technique, termed free‐space angular‐chirp‐enhanced delay (FACED), was used to attain superior image quality . With the assistant of FACED technique, megahertz fluorescence and visible‐bandwidth time‐stretch microscopy was realized . Polarization‐division multiplexing technique was used to increase the spatial resolution .…”